The subject invention relates to reinforcements for high temperature composites that are chemically stable and mechanically stable. In particular, the invention relates to SiC-based reinforcement materials coated with a refractory rare earth boride coating that contributes to the oxidation resistance and high temperature performance of the resulting composite material.
Traditionally many commercial applications of high temperature materials have been filled by Ni-,Co-, and Fe-based metal alloys. These materials function well at temperatures below about 800.degree. C., but rapidly lose strength upon exposure to higher temperatures. Thus, in the field of high temperature materials, researchers have focused on the use of heat resistant fibers to reinforce both metallic and ceramic materials. These high strength composites possess a unique combination of high strength, temperature stability, and low density. This allows for their use in materials for aerospace, automotive, and industrial applications.
Silicon-containing materials are known reinforcements for composite materials. These composites potentially possess high toughness levels and good performance characteristics, thereby making them highly suitable for applications which require light-weight structural materials having high elasticity, high strength, shapability, heat stability, electrical conductivity and heat conductivity. These composites are being increasingly investigated for structural applications.
It is known that many fiber-matrix combinations undergo extensive chemical reaction or interdiffusion between the fiber and matrix materials, each of which is likely chosen for the contribution of specific mechanical and/or physical properties to the resulting composite. Such reaction or interdiffusion can lead to serious degradation in strength, toughness, ductility, temperature stability and oxidation resistance. Some changes may result from the difference in the thermal expansion coefficients of the materials.
To compensate for these problems, a variety of coatings have been suggested for reinforcements intended for use in fiber-matrix composites. For example, U.S. Pat. No. 4,340,636 discloses a surface treatment for the formation of a carbon-rich coating on a stoichiometric SiC substrate filament. Similarly, U.S. Pat. No. 4,315,968 discloses coating SiC filaments with a coating of Si-rich SiC.
U.S. Pat. No. 3,811,920 discusses applying a thin layer of TiC to a filamentary substrate having a SiC surface layer. This TiC layer is reported to impart oxidation resistance to the filament and to provide a diffusion barrier between the SiC-surfaced substrate filament and the matrix metals. Boron nitride has also been used as a SiC coating, as in U.S. Pat. No. 4,642,271.
Intermetallic matrix materials have experienced problems similar to those enumerated hereinabove when combined with reinforcements to produce high performance composites. The problems being experienced in this technology field are generally a result of the fact that the matrix material technology and fiber technology have evolved independent of one another, resulting in chemical and mechanical incompatibility of the precursor materials used to produce composites of the type disclosed hereinabove. The foregoing citations demonstrate various attempts within the field to overcome the inherent shortcomings of these composites by using coating materials to provide the needed characteristics or compatibility.
However, composite materials which have employed techniques and coatings such as the foregoing nonetheless remain limited for high temperature application by concerns regarding the thermomechanical stability, thermochemical stability, oxidation resistance and high temperature fatigue resistance encountered in atmospheric conditions at temperatures above 800.degree. C. A specific problem encountered with a number of these coatings relates to the chemical reactivity of the coating with the matrix materials, which manifests itself in the failure of the mechanical and physical performance of the material in high temperature environments.
Accordingly, an object of the subject invention is to provide a coating for SiC-based reinforcements which permits the use of the reinforcement in composite materials for use at high temperatures above 800.degree. C.
Another object of the invention is to provide a coating for SiC-based reinforcements which prevents chemical reaction between the fiber and the matrix in composite materials.
A further object is to provide a metal matrix composite which contains coated SiC-based reinforcements which maintains high strength and toughness and resists oxidation at high temperatures.